Anatomical Study On The Vegetative Organs Of Two Endemic Taxa In Southeast Asia

The two taxa,Borthwickiaceae and Stixis,were the members of Capparaceae in the traditional classification.In recent years,they have been excluded from Capparaceae,and placed in the GRFT clade of the core Brassicales based on molecular systematics,which are supported by palynology,leaf epidermis micromorphology and embryology.However,their classification status has been kept controversial.To solve this problem,the stem and leaf of Borthwickiaceae and the three species of Stixis(S.suaveolens,S.ovata and S.scandens)were studied by using four methods,i.e.paraffin section,semi thin section,tissue segregation method and scanning electron microscopy(SEM).We have discussed the taxonomic value of the anatomical structure of the stems and leaves.Moreover,we have also analyzed the ecological significance of anomalous structure of stem,and adaptation to drought of leaves in Stixis.The research results are as follows:(1)The exclusion of both Borthwickiaceae and Stixis from Capparaceae are supported.The two taxa,Borthwickiaceae and Stixis,differ markedly from Capparaceae in the stem anatomical characters,including multiseriate pith ray,no uniseriate ray,vestured pits on vessel wall and anomalous structure successive cambia.So,we supported the exclusion of both Borthwickiaceae and Stixis from Capparaceae.(2)Recognition the new family of angiosperms—Borthwickiaceae.Our results showed that Borthwickia is different from Resedaceae in the stem anatomical characters,such as diffuse-porous wood,vestured pits on vessel wall,mostly vasicentric axial parenchyma,and multiseriate rays.Therefore,it is not appropriate that APG IV placed Borthwickia in Resedaceae,and we supported the recognition of Borthwickia at family rank.(3)Support the establishment of Stixaceae.The tribe Stixeae has many stem anatomical features different from Resedaceae,such as successive cambia,diffuse porous wood,vestured pits on vessel wall,vasicentric and diffuse axial parenchyma,and multiseriaterays.Moreover,the tribe Stixeae is distinguished from Resedaceae in the leaf anatomical characteristics,such as whorled vascular bundle,arched vascular tissue with invaginated ends,medullary main vein,mesophyll tissue with stone cells.So,we supported that the tribe Stixeae should be elevated the family rank—Stixaceae.(4)Many structure features accommodated to climbing habits shared by Stixis.For adapting to the climbing habits,some structural changes of Stixis occurred,such as successive cambia to increase the flexibility of stems;wide vessel diameter and simple perforated plate to improve water transport efficiency;multiseriate pith rays and vasicentric axial parenchyma to protect the transfusion tissue and repair damage;vasicentric tracheid,vessel with vestured pits and rich starch grains in pith ray cells to deal with embolism.To suit their habits of climbing,the above structures have important ecological significance in reducing support without reducing their transport capacity.(5)Stixis has some xeromorphic structural features.Through the observation of the anatomical structure of the leaves of Stixis,some xeromorphic characteristics were found,such as developed collenchyma,pith,complex epidermis and rich epidermal hairs.